Production of compressed pellets or tablets from finely divided solid materials



Patented June 17, 1958 PRODUCTION OF COMPRESSED PELLETS OR TABLETS FROMFINELY DIVIDED .SOLID MATERIALS Peter William Reynolds and EricBillingsley Bates, Norton-on-Tees, England, assignors to ImperialChemical Industries Limited, London, England, a corporation of GreatBritain Application November 7, 1955 Serial No. 545,558

Claims priority, application Great Britain November 26, 1 954 5 Claims.(Cl. 18-55) No Drawing.

The present invention relates to the production of compressed pellets ortablets from finely divided solid materials;

i The usual method of producing compressed pellets or tablets fromfinely divided materials is to feed the material into dies and. compressit between punches, one of which, usually the lower one, is adapted toeject the compressed pellet and to return to its position for receivingthe next charge of material.

Difficulty however, may arise due to a particular material inducing,during compression, excessive friction at the inner surfaces of the diesand thereby causing a large proportion of the applied pressure to beconsumed Without forming a satisfactory pellet.

Likewise difficulty often arises in ejecting the compressed pellets fromthe diesowing to the frictionand/ or adhesion between them and the innersurfaces of the dies,

with the result that imperfect or fractured pellets are produced.

In attempts to overcome these difficulties a practice is to mix alubricant with the material tobe pelleted. Lubricants which have'beenused include finely divided long chain aliphatic hydrocarbons,-alcohols,acids, esters It has now been found that these disadvantages may besubstantially overcome by the method of the present invention.

According to the present invention there is provided 8 a process for theproduction of compressed pellets by feeding finely divided solidmaterial into dies and compressing it between punches which comprisesthe successive steps of producing from each die at least one pellet of amaterial comprising a lubricant and then producing from each die atleast one pellet of the desired material.

The number of pellets of the material comprising a lubricant to beproduced from each die before producing pellets of the desired materialwill depend on the behaviour of the respective materials duringpelleting, the amount and effectiveness of the lubricant in thefirstmentioned material and also on the number of pellets of the desiredmaterial it is wished to produce before again producing pellets of thematerial comprising the lubricant. g

It will be understood that preliminary trials will be necessary todetermine the'requisite data for any particular material or mixtures ofmaterials with a desired lubricant.

It has been found that in pelleting some materials satisfactory resultsmay be obtained when employing as little as /2% by Weight of lurbicantin the first-mentioned material, particularly if it is pelletedalternately (that is one for one) with thedesired material.

A particular advantage of the process of the present invention is thatless energy is consumed in overcoming the frictional resistances betweenthe inner surfaces of the dies and the desired material and inconsequence satisfactory pellets may be obtained with pressures muchless than those required by the known method of mixing a lubricant withthe desired material. ,It also follows that pellets of greater strengthmaybe produced at pressures higher than those permissible with the knownmeth- 0d, 01' the volume of desired material pelleted per unit time by agiven machine may be substantially increased as larger pellets may beproduced by the total pressure than can be exerted by the machine. Forexample it has been found that a machine, whose capacity is limited to/8 diameter x A; long pellets when fed continuously with materialcontaining asmall percentage of graphite as lubricant, will producesatisfactory diameter X %s" long pellets when each die is fedalternately with the same material containing graphite as lubricant andwith similar material Without graphite. Although the number of thelarger pellets of the material containing no admixture of graphite isonly one half of the total produced in a given time, their total volumeis in the ratio of 27zi6 compared With the number of the smaller A2"diameter x /s" long pellets that could be produced in the same time.

When the material to be pelleted is abrasive and/or requires highpressures to produce satisfactory results, it has been found preferableto pellet the desired material alternately with that comprising alubricant. For example in preparing catalyst pellets comprising tungsticacid,

nominally each 7 diameter x long from granulated'tungstic acid ofparticle size of 1.4 min, it has been found that satisfactory pelletsare obtained when the desired material ispelleted alternately withsimilar material which has been granulated after the addition of about15% of powdered graphite as lubricant, and that the pressure required isthen much less than that required to produce similar pellets by theknown method of mixing the graphite with the desired material.

in the process of the present invention the pellets of the materialcomprising the lubricant may be crushed and re-used many times, ifdesired after the addition of fresh material and/ or lubricant.

The process of the present invention is further illus trated in thefollowing Tables 1 and 2 of test results obtained from A diameter x 71long pellets of three powdered materials which may be used as catalystsand which for convenience are designated A, B. and C:

The data given in Table 1 are for the purpose of com parison with thosegiven in Table 2 and are for pellets produced by the continuouspelleting of the materials mentioned in the first column when admixedwith the percentage by Weight of the lubricant given in the secondcolumn of the table. The percentages of lubricant shown are those whichhave been found desirable in admixture with the given materials in orderto limit to a reason- I ablevalue the-proportion of the punch pressureWhich'is absorbed in friction while, at the same time, obtaining pelletsof good strength. The fourth column of Table 1 gives the mean percentageof the punch pressure which was absorbed in friction and the fifthcolumn gives the mean vertical crushing strength in :pounds of thefinished pellets. I g

In Table 2 the second column gives the percentage by weight of lubricantwhich was used with thepowdered material in the first column to produceone pellet from each die of the machine before producing pellets fromthe material without added lubricant.

It will be observed from the first entry for each material in Table 2that thelubricant material per se was used as the initial pellet for oneset of pellets of the lubricant-free material. It is to be understoodthat after completing each set of pellets the dies and punches of themachine were cleaned before the next set was produced.

The columns of Table 2 which are numbered from 1 to 12 give, in eachinstance, the number of satisfactory pellets produced from thelubricant-free material, their mean vertical crushing strength inpounds, and the percentage of the punch pressure which was absorbed infriction.

4 at the lower punch pressure used with as much as 3% by weight ofgraphite in the material for the initial pellet.

We claim:

1. A process for the production of compressed pellets by feeding finelydivided solid material into dies and compressing it between puncheswhich comprises the successive steps of producing from each die atleastone pellet of a material comprising a lubricant and then producing fromeach die at least one pellet of the desired material.

2. A process as claimed in claim 1 in which the lubricant is graphite.

3. A process for the production of compressed catalyst pellets oftungstic acid which comprises the steps of feeding finely dividedtungstic acid, substantially of the formula H WO in admixture with atleast /2% by weight of graphite, into dies and compressing it betweenpunches to produce atleast one pellet from each die and thereafterfeeding finely divided tungstic acid free from graphite into the diesand compressing it between punches to produce at least one pellet fromeach die.

4. A process for the production of compressed catalyst pellets of amixture of ferric oxide, chromic oxide and Table I alumina in the ratioby weight of 90:8:2 which comprises the steps of feeding the finelydivided mixture, in Punch Friction, M.V.C.S Material Lubricant pressure,percent (m) admixture with atleast 3% by wcight of graphite, intotons/sq.in. dies and compressing 1t between punches to produce at leastone pellet from each die and thereafter feeding simi- A" 1% graphite. 8020 650 lar finely divided mixture free from graphite into the dies iazgfgglgtg: 28 it 31 8 and compressing it between punches to produce atleast one pellet from each die. I

Table 2 Punch 7 Mean vertical crushing strength in lb. and Friction aspercent load for Pellet Nos.- Material Lubricant, Pressure,

graphite tOnS/Sq'm' 1 2 3 4 V s 6 7 a 9 10 11 12 Percent 100 so 9g 5 so22 A 3 so ig 1 7 so Z2 0.5 so ig 100 so 2 3 oo 23 6 3% 100 so fig 10 6o2?, 4 560 1 so 9g From Table 2 it is clear that the percentage of thepunch pressure absorbed in friction was generally less and that the meanvertical crushing strength of the pellets was higher for those preparedin accordance with the invention than for those produced when using theknown procedure of admixing lubricant with the material as shown inTable 1.

It is also clear from Table 2 that if the percentage of the punchpressure absorbed-in friction as given in Table 1 is taken as the basisfor satisfactory operation according to the invention, three or fourpellets of material A may be produced from each die after producing aninitial pellet of the materialrwith as little as /2.% by weight ofgraphite mixed therewith, but that with material C at least 1% by weightof graphite is required for similar results at the lower punch pressureused and that with material B only one or two pellets can be producedReferences Cited in the file of this patent UNITED STATES PATENTS1,289,570 Stokes Dec. 31, 1918 2,262,155 Andrews Nov. 11, 1941 2,536,758Reynolds et a1 Jan. 2, i

1. A PROCESS FOR THE PRODUCTION OF COMPRESSED PELLETS BY FEEDING FINELYDIVIDED SOLID MATERIAL INTO DIES AND COMPRESSING IT BETWEEN PUNCHESWHICH COMPRISES THE SUCCESSIVE STEPS OF PRODUCING FROM EACH DIE AT LEASTONE PELLET OF A MATERIAL COMPRISING A LUBRICANT AND THEN PRODUCING FROMEACH DIE AT LEAST ONE PELLET OF THE DESIRED MATERIAL.